Chloroquine Affects Frequency-Dependent Endocytosis in Diaphragm Muscle of Old Mice

Abstract

Chloroquine is an antimalarial agent commonly used as an autophagy inhibitor by preventing autolysosomal fusion. Autophagy is an important cellular process through which defective cytoplasmic structures are recycled and degraded that has been implicated in aging neuromuscular effects. We recently showed that acute chloroquine treatment blunts the maximal forces generated by the diaphragm muscle, likely by impairing neuromuscular transmission generally consistent with aging effects in the diaphragm muscle. In addition, old mice show increased accumulation of autophagy proteins in phrenic motor neurons, consistent with impaired autophagy flux. In the present study, we hypothesized that chloroquine treatment results in accumulation of bulk endosomes pre-synaptically. Each hemidiaphragm with attached phrenic nerve was excised from 24-month-old male and female C57BL/6 mice to evaluate frequency dependent (0 vs 80 Hz stimulation) uptake of the styryl dye FM4-64, given that 80-Hz stimulation frequency induces bulk endocytosis after synaptic vesicle retrieval. Diaphragm-phrenic nerve preparations were incubated in 5 μM FM4-64 for 1 h prior to 6-min nerve stimulation (0.5 ms supramaximal pulses in 80 Hz, 200 ms trains delivered at 40% duty cycle) followed by a 5-min post-stimulation incubation. FM4-64 uptake at diaphragm neuromuscular junctions (NMJs) was measured using confocal microscopy. Preparations were randomly assigned to co-incubation with chloroquine (50 μM) or vehicle (Rees-Simpson solution). There was minimal FM4-64 uptake in the absence of phrenic nerve stimulation, consistent with the activity-dependent uptake of the styryl dye. With 80 Hz stimulation, high levels of FM4-64 uptake were evident in both the chloroquine and vehicle-treated groups without significant differences across groups. FM4-64 fluorescence was evident throughout the presynpatic terminal with few areas of dense accumulation, consistent with retention of endosomes. There was no evidence of greater endosome accumulation with chloroquine treatment. Of note, a slight increase in FM4-64 uptake was evident in the chloroquine treated groups compared to vehicle in the non-stimulated NMJs. These results are consistent with FM4-64 uptake reflecting bulk endocytosis that mainly occurs during high frequency stimulation. The impairment in autophagy flux following chloroquine treatment is expected to result in endosome accumulation but it is possible that the stimulation protocol was insuffcient to elucidate this possibility. Collectively, the findings of this study are consistent with impaired autophagy at diaphragm presynaptic terminals which may underlie neuromuscular deficits observed in old age. Supported by NIH grant R01 AG057052 and the Mayo Clinic. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.